Glide height testing using a glide head apparatus with a piezoelectric actuator

ABSTRACT

A glide head apparatus for testing surface characteristics of a disc includes a gimbal, a slider and an actuator including a piezoelectric material disposed between the gimbal and the slider. A voltage applied across the piezoelectric material is controllable to cause the piezoelectric material to expand or contract depending on the applied voltage so as to vary the fly height between the slider and a disc under test. The fly height can be varied without substantially varying the linear velocity of the disc under test. Methods of testing surface characteristics of a disc using the glide head apparatus also are disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional ApplicationSerial No. 60/082,232, filed Apr. 16, 1998. This application is relatedto a concurrently-filed application Ser. No. 09/252,889 entitled "GlideHead Apparatus For Testing Recording Media," assigned to the assignee ofthe present invention and incorporated herein by reference in itsentirety.

BACKGROUND

The present invention relates generally to magnetic recording media and,in particular, to apparatus and techniques for testing the glide heightcharacteristics of magnetic recording media.

Disc drives are the primary devices used for mass storage of computerprograms and data. Within a disc drive, a load beam supports ahydrodynamic air bearing slider close to a rotating magnetic disc. Theload beam supplies a downward force that counteracts the hydrodynamiclifting force developed by the slider's air bearing. During operation,the magnetic head rides at a distance from the surface of the magneticdisc. That distance must be small enough to allow high density recordingwhile preventing damage that would otherwise be caused by contactbetween the spinning disc and the magnetic head.

High areal densities currently are achieved by reducing the separationbetween the disc and the head to less than twenty nanometers (nm).However, some level of disc roughness is required to reduce adhesiveforces when the head is at rest. The level of disc surface topographymust, therefore, be kept within a tight range to fly the head safely atlow altitudes while simultaneously preventing it from sticking to thedisc surface when the head is at rest. Thus, the topography of the discsurface is critical to the proper operation of the disc drive.

As part of the process of manufacturing hard files, the quality of amagnetic disc is provided by determining the glide conditions which canbe established between the disc and a glide head. In particular, theeffect of outwardly projecting defects on the surface of the magneticdisc is. studied during glide height testing. When such defects arelarge enough to close the gap between the magnetic disc and the glidehead, the defects strike the glide head. The movement of the glide headcan be sensed, for example, by a sensor such as a piezoelectrictransducer, which generates an electrical signal indicating the adjacentpassage of an outwardly projecting defect.

During testing, a gliding action is brought about as a layer of air,dragged along by the spinning disc surface, is compressed between thesurface of the disc and the adjacent surface of the glide head. As aresult of the gliding action, the glide head rides at a distance fromthe surface of the disc. That distance is referred to as the "fly"height of the glide head and is determined, in part, by the peripheralspeed of the rotating disc and the air pressure surrounding the disc.Thus, the fly height of the glide disc can be varied by changing thespeed at which the disc rotates. A glide avalanche breaking point(GABP), which is used by engineers to characterize the surface of thedisc, can be obtained based on the interaction between the disc surfaceand the glide head at different fly heights.

Several difficulties arise, however, when the linear velocity of thedisc is varied to obtain a measure of the glide avalanche breakingpoint. The impact energy which is detected by a sensor depends on thevelocity and, in some cases, is approximately proportional to the squareof the velocity. Thus, it can be difficult to interpret the signalsreceived by such a sensor. Furthermore, changing the linear velocity canaffect the pitch and roll of the glide head. That, in turn, can affectthe level of interference detected by the sensor. Additionally, therelationship between fly height and linear velocity may not be linear atvery low speeds, such as speeds less than 200 inches per second, makingit difficult to correlate the velocity with fly height.

SUMMARY

In general, glide head apparatus and methods for testing surfacecharacteristics of a disc, such as a magnetic disc, are disclosed. Thetechniques can be used, for example, to perform glide tests in which thefly height is varied while the linear velocity of the disc remainssubstantially constant.

According to one aspect, the glide head apparatus includes a gimbal, aslider and an actuator including a piezoelectric material disposedbetween the gimbal and the slider. A voltage applied across thepiezoelectric material is controllable to cause the piezoelectricmaterial to expand or contract depending on the applied voltage so as tovary a distance, such as the fly height, between the slider and a discunder test.

Various implementations include one or more of the following features.The voltage across the piezoelectric material can be controlled by adigital signal processor. The voltage applied across the piezoelectricmaterial can be controlled to vary the fly height of the glide headapparatus without substantially varying the linear velocity of the discunder test. In some cases, the piezoelectric material is expandable overa range of at least about 0.010 inch.

The piezoelectric material can comprise a ferroelectric material, suchas lead zirconium titanate. Other piezoelectric materials also can beused. In some implementations, multiple piezoelectric actuators arecoupled in series. The voltages across the respective piezoelectricactuators can be controlled to provide a corresponding change in the flyheight of the glide head apparatus.

In some embodiments, the glide head apparatus also includes an arm witha load arm, and an actuator for positioning the arm over the disc. Thegimbal can be attached to the load arm. The apparatus also can include atransducer for sensing interactions between the slider and the discunder test.

According to another aspect, a method of testing surface characteristicsof a disc includes causing the disc to rotate at a predetermined linearvelocity with a glide head positioned at an initial fly height above asurface of the disc. The method further includes acquiring dataindicative of interactions between the glide head and the disc while thedisc is rotating at the predetermined linear velocity. The voltageacross a piezoelectric actuator is changed to cause a correspondingchange in the fly height between the glide head and the surface of thedisc. The act of acquiring data can be repeated once the fly height hasbeen changed.

One or more of the following features are present in someimplementations. The act of changing the voltage across thepiezoelectric actuator can cause a reduction in the fly height.Furthermore, the acts of changing the voltage and acquiring dataindicative of interactions between the glide head and the disc can berepeated until sufficient information is acquired to determine a glideavalanche breaking point for the disc. Acquiring data at different flyheights can be performed while the disc rotates at a substantiallyconstant linear velocity. The acquired data can be used to determinewhether the disc passes the glide test.

Various embodiments include one or more of the following advantages.Using a substantially constant linear velocity as the fly height isvaried during the glide test makes it easier to interpret the signalsindicative of the interaction between the glide head and the disc. Forexample, the effect of unknown variables, such as the effect thevelocity has on the impact energy between the disc and the glide head,can be removed or reduced. Additionally, using a substantially constantlinear velocity can reduce the effects on the pitch and roll of theglide head that a changing velocity may cause.

Other features and advantages will be readily apparent from thefollowing detailed description, the accompanying drawings, and theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a load arm supporting a head gimbal assemblyembodying the present invention.

FIG. 2 is a perspective view of the load arm and head gimbal assembly ofFIG. 1.

FIG. 3 is a simplified side view of the head gimbal assembly including apiezoelectric actuator according to the invention.

FIG. 4 is a partial side view illustrating additional details of thehead gimbal assembly.

FIG. 5 is a block diagram illustrating a control system for thepiezoelectric actuator.

FIG. 6 is a flow chart of a method according to the invention.

FIG. 7 illustrates another embodiment of the head gimbal assembly withmultiple piezoelectric actuator in series according to the invention.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, a glide head assembly includes an actuator,arm 20 connected to an actuator 22 and supports a head gimbal assembly(HGA) 24 over a magnetic disc 26. The actuator 22 positions the arm 20along an arc 28 over the magnetic disc 26. The arm 20 includes asupporting arm 30, a base plate 32, and a load arm 34. The HGA or glidehead 24 includes a gimbal 36 and a slider 38. The actuator 22 rotatesthe arm 20 to position the slider 38 along the arc 28.

The gimbal 36 is welded to the load arm 34 and resiliently supports theslider 38 and allows it to pitch and roll while it follows thetopography of the rotating disc 26. The slider 38 includes a self-actinghydrodynamic air bearing surface which can take the form of multiplerails 44 with tapered forward surfaces. The rotating disc 26 forces airinto the tapers and produces pressure beneath the rails 44 resulting inthe air bearing surface.

The load arm 34 is compliant in the vertical axis to allow the slider 38to follow the topology of the disc 26, and is rigid in the in-plane axesfor precise positioning of the slider. The load arm 34 also supplies adownward force that counteracts the hydrodynamic lifting force developedby the slider's air bearing.

During the glide test process, if contact occurs between the glide head24 and a disc asperity or a defect, the glide head vibrates and isdeformed slightly. The deformation of the slider 38 results indeformation of a piezoelectric transducer 60 (FIG. 4) which can bemounted on an extended wing of the trailing edge of the slider 38. Apotential difference is generated between the electrodes of thetransducer, and the transducer generates an electrical signal indicatingthat the glide head has struck the surface of the rotating disc 10.Electric signals generated by the transducer are provided topre-amplification circuitry 62 and a band pass filter 64 via smalltwisted copper wires 66. A digital data acquisition system 68 processesthe filtered data to compute the GABP and to determine whether the discpasses or fails the glide test.

The glide head assembly 18 also includes a piezoelectric actuator 48disposed between the gimbal 36 and the slider 38 (FIG. 3). Thepiezoelectric actuator 48 can be attached to the gimbal 36 and theslider 38 using, for example, an epoxy glue. In one implementation, thepiezoelectric actuator 48 comprises a ferroelectric material such aslead zirconium titanate (PbZrTi₃), also known as PZT.

The crystal structure of PZT contains inherent asymmetries in itslattice. In general, a material comprising PZT can be deposited on awafer to a thickness on the order of one micron, for example, by a solgel process. The PZT material then is annealed at a high temperature toform a crystalline lattice. Finally, the PZT material is poled to inducethe desired piezoelectric properties. In other words, the randompolarized crystal orientations in the ceramic are permanently aligned byapplication of a strong electric field. Once the material is poled, thePZT film will expand piezoelectrically when a voltage of one polarity isapplied across it and will contract when a voltage of the oppositepolarity is applied. Other piezoelectric materials also can be used forthe actuator 48, including, for example, PbTiO₃ and PbZrO₃.

With respect to the piezoelectric actuator 48, one electrode can beattached to the underside of the slider 36 which can comprise aconductive material. The backside surface of the actuator 48 is slightlylarger than the gimbal 36 so that a second electrode can be attached tothe backside surface. When a voltage of a first polarity, for example, anegative voltage, is applied, the actuator expands so that the glidehead moves closer to the surface of the disc 26. On the other hand, whena voltage of the opposite polarity, for example, a positive voltage, isapplied, the actuator 48 contracts so that the glide head moves awayfrom the surface of the disc 26. Thus, by controlling the voltageapplied to the piezoelectric actuator 48, the vertical position of theslider 38 can be controlled precisely. In general, the expansion orcontraction of the piezoelectric actuator 48 and, therefore, thedisplacement of the slider 38 in the vertical direction corresponds tothe applied voltage. In this way, the fly height of the glide head overthe surface of the disc 26 can be varied without changing the rotationalspeed of the disc.

The range of variation in the contraction and expansion of thepiezoelectric actuator 48 should be at least about 10 to about 20percent of the "Z-height" which refers to the vertical distance from thebase of the mounting plate 32 to the upper surface of the disc 26. Forexample, if the Z-height is approximately 100 mils (0.100 inch), theactuator 48 should be capable of contracting and/or expanding over arange of at least about 10 to 20 mils (0.010 to 0.020 inch).

Referring to FIG. 5, a processor 50, such as a programmable digitalsignal processor (DSP), is programmed to control the operation of thepiezoelectric actuator 48. Alternatively, the processor 50 can include acentral processing unit, a general purpose or special purpose processor,a minicontroller, a microprocessor or a microcomputer for controllingoperation of the piezoelectric actuator 48. Various types of memory areassociated with the processor 50, including read only memory (ROM) 52and random access memory (RAM) 54. The processor 50 provides outputsignals which are representative of the magnitude and polarity of thevoltage to be applied to the piezoelectric actuator 48. Those signalsare provided to a digital-to-analog (D/A) converter 56. The output ofthe D/A converter 56 is provided to an amplifying circuit 58. Theamplifying circuit 58 then provides a voltage which is applied acrossthe piezoelectric actuator 48.

In operation, a disc 26 under test is rotated at a predetermined linearvelocity so that the slider glide head is positioned at an initial flyheight h₁ above the surface of the disc 26. The linear velocity of thedisc 26 can be selected to coincide approximately with the expected GABPof the disc. For example, for a sample disc having a GABP of about 1.0micro-inch, a glide write 11-mil head having a fly height of about 1.0micro-inch can be used, with the disc 26 spinning at a linear velocityof about 400 inches per second. Output signals from the piezoelectrictransducer are obtained at the initial fly height h₁. The voltage acrossthe PZT actuator 48 then is adjusted to cause the actuator to expand inthe vertical direction by a predetermined amount. As the actuator 48expands in the vertical direction, the slider 38 is brought closer tothe surface of the disc 26 by a corresponding amount. The linearvelocity of the disc 26, however, remains substantially constant. Again,output signals from the transducer are obtained which correspond to asecond fly height h₂. The process is repeated such that the fly heightof the HGA 24 is changed by varying the voltage across the PZT actuator48, and the interaction between the GHA and the disc 26 is sensed by thetransducer. The process is continued until sufficient data has beencollected to determine the glide avalanche breaking point of the disc 26or until the fly height reaches a predetermined minimum value.

The linear velocity of the disc 26 can remain substantially constanteven as the fly height of the GHA 24 is varied. Using a substantiallyconstant linear velocity as the fly height is varied during a glide testmakes it easier to interpret signals from the transducer 60 by removingcertain unknown variables such as the effect that the velocity has onthe impact energy between the disc 26 and the slider 38. Additionally,using a substantially constant linear velocity can reduce the effects onthe pitch and roll of the glide head that a changing velocity may cause.

Although the embodiment discussed above incorporates a singlepiezoelectric actuator 48, in other implementations two or morepiezoelectric actuators 48A, 48B and 48C (FIG. 7) can be provided inseries between the gimbal 36 and the slider 38. By controlling thevoltage applied across each of the piezoelectric material in each of theactuators 48A, the actuators either expand or contract by a desiredamount, thereby changing the fly height of the glide head.

Other implementations are within the scope of the following claims.

What is claimed is:
 1. A method of testing surface characteristics of adisc, the method comprising:causing the disc to rotate at apredetermined linear velocity with a glide head positioned at an initialfly height above a surface of the disc; acquiring data indicative ofinteractions between the glide head and the disc while the disc isrotating at the predetermined linear velocity; changing a voltage acrossa piezoelectric actuator to cause a corresponding change in the flyheight between the glide head and the surface of the disc; and repeatingthe act of acquiring data once the fly height has been changed.
 2. Themethod of claim 1 wherein the act of changing the voltage across thepiezoelectric actuator causes a reduction in the fly height.
 3. Themethod of claim 1 wherein the acts of changing the voltage and acquiringdata indicative of interactions between the glide head and the disc arerepeated until sufficient information is acquired to determine a glideavalanche breaking point for the disc.
 4. The method of claim 1 whereinacquiring data at different fly heights is performed while the discrotates at a substantially constant linear velocity.
 5. A method oftesting surface characteristics of a disc, the method comprising:causingthe disc to rotate at a predetermined linear velocity with a glide headpositioned at an initial fly height above a surface of the disc;acquiring data indicative of interactions between the glide head and thedisc while the disc is rotating at the predetermined linear velocity;changing a voltage across a piezoelectric actuator to cause a reductionin the fly height between the glide head and the surface of the disc;repeating the act of acquiring data once the fly height has beenchanged; and repeating the acts of changing the voltage and acquiringdata until sufficient information is acquired to determine a glideavalanche breaking point for the disc.
 6. A glide head apparatus fortesting surface characteristics of a disc, the apparatus comprising:agimbal; a slider; and a plurality of actuators coupled in series, eachof the actuators including a piezoelectric material disposed between thegimbal and the slider, wherein a voltage applied across thepiezoelectric material of each actuator is controllable to cause thepiezoelectric material to expand or contract depending on the appliedvoltage so as to vary a fly height between the slider and a disc undertest.
 7. The apparatus of claim 6 wherein the voltage applied across thepiezoelectric material is controllable to vary the fly height of theglide head apparatus without substantially varying a linear velocity ofthe disc under test.
 8. The apparatus of claim 6 wherein the voltageacross the piezoelectric material is controlled by a digital signalprocessor.
 9. The apparatus of claim 6 wherein the piezoelectricmaterial comprises a ferroelectric material.
 10. The apparatus of claim6 wherein the piezoelectric material comprises lead zirconium titanate.11. The apparatus of claim 6 further comprising:a transducer for sensinginteractions between the slider and the disc under test.